The growth of macroalgal mats on the Ythan Estuary, with respect to their effects on invertebrate abundance
Changes in the nutrient loading to the Ythan estuary over the past twenty-five years were examined by analysing available long-term data sets to attempt to correlate nutrient changes with reported increases in the growth of opportunistic green macroalgae in the estuary. Analysis revealed a three-fold increase in nitrogen and a two-fold increase in phosphorus as a result of increases in agricultural run-off and sewage effluent respectively. Concurrent with these nutrient increases, the peak summer biomass of Enteromorpha on Newburgh South Quay Mudflat increased from around 1kg FW./m2 in the mid 1960's to 3kgFW./m2 in 1985/86, with evidence for further increases in the biomass and distribution of other opportunistic species such as Ulva and Chaetomorpha elsewhere in the estuary. Much of the primary production was found to become incorporated into the sediment, through burial of macroalgae in autumn. The effects of dense accumulations of algae on the underlying macrofauna were investigated by means of a field experiment. Corphium was shown to be intolerant of algal cover, but the abundance of Macoma, Nereis and Capitella increased. Algal cover was also shown to cause sediment anoxia and lead to accretion of silt. A further field experiment was carried out to investigate the processes by which algal cover affected macrofaunal abundance. Both the physical and organic (biological) effects of algal mats were shown to be important in determining invertebrate abundance, mostly as a result of changes in sediment chemistry. A tidal multi-microcosm system was designed and constructed to conduct laboratory based studies, but treatment effects were found to be enhanced compared to field conditions, such that results could not easily be extrapolated to the natural environment. It was recommended that monitoring of water nutrient, concentrations together with algal biomass and distribution, be continued, as changes in macrofaunal abundance under algal mats may create food shortages for certain shorebirds and fish.